1. Field of the Invention
The present invention relates to a keyboard scanning circuit, and, more particularly, to an improvement over the conventional two-dimensional telephone keyboard scanning circuit which provides the maximum number of keys available in a telephone system with a given number of pins and to integrate other functions thereon to be selected by means of diodes.
2. Brief Description of the Prior Art
A conventional two-dimensional keyboard scanning circuit of a telephone system has a configuration that data ports thereof are divided into two dimensional ports, i.e. the row ports and the column ports, to correspond to a two-dimensional matrix. In the two-dimensional keyboard configuration, a key switch is available in the interconnection between any one of the row ports and any one of the column ports, as shown in FIG. 1. Such a conventional keyboard configuration has a limitation in the maximum number of keys that are available for no key is arranged on the interconnection between any two ports of the same set, for example between two columns or between two rows.
As shown in FIG. 1, the keyboard scanning circuit of the conventional two-dimensional keyboard configuration, which is designated with reference numeral 1, comprises m columns (C1, C2, . . . , Cm) and n rows (R1, R2, . . . , Rn). In accordance with the conventional two-dimensional keyboard arrangement, a key switch may be provided between each of the rows and each of the columns, namely in mathematical form between the ith column (1.ltoreq.i.ltoreq.m) and the jth row (1.ltoreq.j.ltoreq.n). Hence, the maximum number of keys available in the two-dimensional keyboard configuration is m.times.n and no key is available between any two rows or between any two columns. However, since actually, the rows and columns have no difference from each other, it is a waste for not being able to provide a key between any two rows or any two columns.
Furthermore, in the conventional two-dimensional keyboard configuration, a 3.58 MHz oscillator is usually adopted to provide the system frequency of the keyboard scanning operation. In detecting the actuation of a key, all the columns are set to for example a logic high level and the rows are set to logic low level so that a detection of the change of output from high to low occurring at one of the columns indicates the associated key is depressed or actuated. Using a 3.58 MHz oscillator to provide the system frequency consumes dissipates much more power than using a 32768 Hz oscillator and it is about 100 times more power dissipation. This is also a serious problem to be overcome.